Apple Support Discussions user "Carms" reports:
I have been having this issue as well, for a few months now it will start up automatically, sometimes at night, sometimes during the day. Quitting usually results in it launching again, and after a few seconds to minutes it starts with whatever is on the top of the open window in itunes. And then it stops mid song, and starts again.Other posts on this thread share this sentiment. Early suggestions pointed to third-party programs like Boxee or last.fm causing the issue, though other users reported having no extras for iTunes added. ASD user "Carms" suggests a workaround:
There are a few threads on this board describing this as far back as august of 2009. None of the fixes listed in any of these threads proves a permanent fix. It's not last.fm, it's not iTunes Helper in the login items. It's not the CD/DVD pref pane settings. It doesn't seem to be the shockwave flash plugin, since uninstalling doesn't fix it, although the error messages on the console are more frequent with it installed. But, it just launched while i typed this message and there's nothing new on the console.
Just a tip for those of who who are going crazy with this: make a blank playlist, select the playlist in the left column and close itunes, or just keep it open. At least music won't randomly play.We will keep an eye on this issue as more reports are sure to come in. As Apple releases updates for iTunes and Mac OS X, there is sure to be a fix.